Go for what you want....
Seat covers are not CHAV at all! They are a cheap way to cover up the crappy colour of the standard interior.....i couldnt stand my brown colour in my old K10.
Spinners are a bad idea though as if you want to attract attension to yourself and your car then this is the way as lowlifes will brake into your car just for the cd player.
Most of those modds your looking at aren't going to affect your insurance....exhuast maybe....but not alot....
A word of advice that my dad told me and that i should have listened to, but didnt when i got my first k10 at 15....so the same as you...is to save the money for the insurance....to insure my k10 when i turned 17 was £1200! AND THATS A 1.0L so just think are you going to be able to pay the deposite and first payment of around £300? i couldnt and had to get my dad to lend me some. Dam you £10 a week paper round! lol
But as i said....just a word of advice 
